Heat can be converted into power by the well known Rankine cycle as follows:                Step 1: pumping a liquid working fluid to an elevated pressure;        Step 2 heating the resulting elevated pressure liquid working fluid by indirect heat exchange against the heat source where said heating results in:                    (a) boiling the working fluid; and            (b) preferably superheating the boil-off to a sufficient degree to ensure the working fluid remains substantially in the vapor state throughout step 3's work expansion step;                        Step 3: work expanding (defined herein as expanding at substantially constant entropy) the resulting heated working fluid in a turbine expander;        Step 4: condensing the work expanded working fluid by heat exchange against cooling water to prepare the working fluid for a new cycle of steps 1 through 3.        
In one variation (hereafter, the supercritical variation such as supercritical steam cycle), the liquid working fluid is pumped to a supercritical pressure (i.e. a pressure above the liquid's critical pressure) in step 1 and heated to a supercritical temperature (i.e. a temperature above the liquid's critical temperature) in step 2.
In another variation, the thermodynamic efficiency of step 3's work expansion step is increased by using a multi-stage expander where the working fluid is re-heated against the heat source between stages.
In another variation, the working fluid is preheated against a low grade heat source prior to boiling the working fluid against a higher grade heat source (See for example U.S. Pat. No. 3,950,949 and U.S. Pat. No. 4,182,127).
The present invention differs from the conventional application of the Rankine cycle in a significant way. In particular, instead of requiring the heat source to be of sufficiently high temperature or “high grade” to boil/superheat the working fluid in step 2 (or heat working fluid to supercritical temperature in case of supercritical cycle), a dense fluid expander is utilized in step 3.
In this fashion, the working fluid is allowed to be a liquid at the end of step 2 (or at least mostly a liquid as there are dense fluid expanders that can tolerate some vapor at the inlet) resulting in an expander discharge at the end of step 3 containing a vapor portion and a (typically bigger) liquid portion. Accordingly, the present invention is suitable for relatively low temperature (typically 100° C. or less) or “low grade” heat sources (often referred to as “waste heat”) that are incapable of boiling/superheating the working fluid in step 2 (or heat heating working fluid to supercritical temperature in case of supercritical cycle). Or at least incapable of providing such an amount of heat while still allowing, as required in step 4, the expanded working fluid to be condensed without any refrigeration beyond ordinary cooling water.
In addition to its applicability to low grade heat, the present invention also avoids the thermodynamic penalty associated with employing a boiling liquid to recover heat. (See for example EP 1389672 which utilizes boiling fluid to recover low grade heat of compression.) In particular, since a liquid (or a least a single component liquid) boils at a constant temperature, the associated heat exchanger has large temperature differences between the hot and cold streams (i.e. very non “tight” cooling curves) which the present invention avoids. (Although the supercritical variation of the Rankine cycle also avoids this thermodynamic penalty associated with employing a boiling liquid to recover heat, if the fluid's critical temperature is below the temperature of the low grade heat source, the liquid condensed at the cooling water temperature is relatively close to the fluid's critical temperature and consequently the pump work will be too high relative to the expander work for the cycle to be efficient.)
Of course, as the skilled practitioner can readily appreciate, there is a thermodynamic (and mechanical complexity) penalty associated with the present invention's work expansion of a gas vis-à-vis the conventional work expansion of a vapor. However, recent advances in dense fluid expanders, coupled with the ever increasing energy costs, are working to justify the present invention's use a dense fluid expander to convert low grade heat sources into power. Examples of such low grade heat include compressor discharge, geothermal sources (such as hot spring) and the heat from solar collectors.
Heretofore, the application of two-phase dense fluid expanders has been limited to refrigeration cycles where, prior to work expanding the working fluid, the working fluid is cooled (e.g. to take advantage of refrigeration producing effect when a fluid is work expanded) instead of heated as in present cycle (e.g. to take advantage of work producing effect when a fluid is work expanded). For example, U.S. Pat. No. 5,564,290 teaches use of a two-phase dense fluid expander in an air separation plant. U.S. Pat. No. 6,763,680 teaches expanding liquid natural gas in a two-phase dense fluid expander. Two-phase dense fluid expanders have also been proposed in a standard vapor compression refrigeration cycle as a replacement for a throttle (Joule-Thompson) valve.